While all eyes were on the glamorous gowns on the red carpet outside of the Metropolitan Museum of Art on Monday night, inside the Costume Institute Gala, it was all about Lady Gaga who stole the show in sheer crystal catsuit. Skipping the red carpet so she could prepare for her mini-concert, Gaga performed four songs in front of a New York City-themed backdrop–while clad in a crystal-encrusted catsuit by Giorgio Armani (who notably designed her Grammys ensembles). The ensemble and the show had the star-studded audience tweeting away: “GaGa singing beautifully at Met Ball. (she’s not the lamp post – underneath on the piano),” joked Jimmy Fallon, while Cash Warren, there with wife Jessica Alba raved about her performance. “Jess and I are front row center. Insane! She’s amazing,” Warren Tweeted, adding, “haven’t heard a voice that wonderful in person in a long time.” Katy Perry joined in on the Gaga lovefest, Tweeting, “I love @ladygaga She’s still cool.” Meanwhile, a Michael Kors-clad Brooke Shields told PEOPLE that the performance was one of the highlights of the night, “I love whenever I can see Lady Gaga.” —Cara Lynn Shultz